This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. During puberty, luteinizing hormone (LH) pulse frequency and amplitude increase overnight, but slow again during the day. Data suggest that estradiol (E2) and progesterone (P) also increase overnight during puberty. We have hypothesized that morning increases in P and E2 help suppress LH pulsatility during the following day. An earlier study demonstrated that E2 administration suppresses nocturnal increases in LH pulsatility during puberty. However, studies in adults suggest that P is the principal effector of LH pulse frequency slowing. It is unknown if P administration can suppresses nocturnal increases in LH pulsatility during puberty. To assess this further, we will study normal early to midpubertal girls. Volunteers will undergo a frequent sampling study in the GCRC. Oral P or placebo will be administered (according to randomization) at 1600 h and 2000 h. Frequent blood sampling from 1900-0900 h will allow assessment of overnight changes in LH pulse frequency after either P or placebo administration.